1. Field of the Invention
The present invention relates to the total recovery of uranium, yttrium, thorium and rare earth values contained in a phosphate-bearing ore over the course of the wet-process preparation of phosphoric acid.
2. Description of the Prior Art
It is known to this art that the phosphate-bearing ores used for the production of phosphoric acid contain not inconsiderable amounts of uranium, yttrium, thorium and the rare earths. With respect to such ores, in the group comprising thorium, rare earths and yttrium, the yttrium values constitute an amount which is approximately one-half of the total amount of such values.
It too is known that, when subjecting a phosphate ore to attack by means of sulfuric acid, the major fraction of the uranium (approximately 95%) is solubilized in the phosphoric acid formed and well-known processes are used for recovering that element, in particular by liquid/liquid extraction, or by precipitation from the phosphoric acid of a secondary uranium-containing gypsum.
However, the major portion of the rare earths and yttrium present in the ore is not solubilized in the attack or acidulation operation, and coprecipitates with the gypsum. The amount of such elements which goes into solution depends upon the nature of the ore and generally ranges from about 5 to 20% of the total amount present in the ore. In order to subsequently recover such elements, the gypsum has to be treated, for example, by washing with sulfuric acid.
The recovery of the uranium values on the one hand and the other elements referred to above, on the other hand, therefore requires two separate treatments, one relating to the phosphoric acid and the other to the gypsum.
Serious need therefore exists in this art for a process which, in a single operation, provides for the combined recovery of all of the uranium values and the aforenoted other elements.
That problem has been partially solved. Indeed, a process is known (British Patent Specification No. 793,801) in which the degree of solubilization of the yttrium and rare earth values is increased, at the moment of the attack phase, by the addition of silica. This provides a solution of phosphoric acid which contains uranium and a portion of the yttrium and rare earths, which is larger than that achieved under the usual attack phase conditions.
However, the addition of silica concomitantly gives rise to a number of disadvantages.
First of all, if the proportion of rare earths and yttrium which is solubilized in the attack operation increases with the amount of silica added, a plateau is nonetheless quickly reached. Thus, it has been found difficult to solubilize more than approximately 40% of the total amount of the elements in question, in the attack operation.
Moreover, the addition of silica interferes with filtration of the attack slurry when separating the gypsum and the phosphoric acid. The greater the amount of silica, the greater the reduction in the rate of filtration. That is a very serious disadvantage from an industrial point of view.
Finally, the silica may be found to give rise to difficulties in the subsequent stages in a process for the production of phosphoric acid, in particular in liquid/liquid extraction operations.